1. Field of the Invention
One aspect of the present invention relates to a method for preparing a brazed surface for receiving a coating.
2. Background Art
Aluminum heat exchangers are commonly utilized in vehicle HVAC systems, for example, radiators, condensers and evaporators. After assembly of the components (e.g. fins, tubes and headers) of the heat exchanger, flux is applied to the surface of the heat exchanger. The flux can be an aluminum potassium fluoride based composition, for example, NOCOLOK(R) Flux, available from Solvay Fluorides, LLC. The unit is then typically brazed in a controlled atmosphere furnace. The flux is utilized to inhibit the growth of an undesirable oxide layer and/or to promote wetting of the liquid metal as it passes through the brazing furnace.
Typically, there is no effort to remove the flux after the brazing operation. Therefore, the flux remains on the surface of the heat exchanger. During the brazing operation, the flux layer is formed. The flux layer includes a conversion coating, which is a coating that is chemically bonded to the brazing surface. The flux also leaves behind a powder substance on the brazing surface. The powder substance, which is similar to a light layer of talc coated onto the surface of the heat exchanger, inhibits effective wetting of the surface and further coating of the surface. Since the powder itself delaminates from the heat exchanger surface over time, a coating over this powder layer is not effective, and becomes easily delaminated.
Post-brazing coatings can serve one or more purposes. One or more coatings can provide odor prevention, watershed, and/or anti-corrosion features.
Several proposals have been advanced to adhere a coating to the flux layer with unsatisfactory results. Coating methods have included application of a conversion coating applied by dip or spray applications. Further, paint-type coating methods have been proposed.
In light of these unsatisfactory results, methods have been proposed for removing the powder component of the flux layer after brazing so that the surface can be effectively coated. Several attempts have been made to use acid etching techniques to remove the flux layer. Non-limiting examples of acids used in accordance with these techniques include hydrofluoric, nitric, hydrochloric and phosphoric acids. Moreover, base etching techniques have been utilized with potassium hydroxide and sodium hydroxide. These methods have not effectively removed the flux layer such that the surface is suitable for application of subsequent coating layers.
In light of the foregoing, what is needed is a method and composition for removing at least a portion of the powder component of the flux layer from a surface such that the surface is suitable for receiving a subsequent coating without substantial delamination.